Well bores typically originate at or near the Earth's surface, and penetrate through one or more layers of the Earth's crust toward a predetermined depth or geologic formation. A variety of instrument systems have been used to estimate the elemental or chemical compositions of formations and formation fluids downhole using an uphole laser and a fiber optic transmission line downhole. Similarly, uphole lasers have been used to measure downhole temperature or pressure using a fiber optic sensor that is located downhole. Semiconductor lasers such as laser diodes are compact enough to be packaged within a tool that is lowered into a well bore. However, commercially available semiconductor lasers dim dramatically with increasing temperature and generally stop lasing altogether above approximately 125° C. Well bore temperatures usually exceed 125° C. and can even reach 200° C. or higher, making such devices unusable without cooling, which adds considerable complexity. To date, no satisfactory measurement system that uses a downhole laser has yet been realized.
While previously known laser instrument systems, for example, those employing optical or electromagnetic energy derived from a gaseous or vaporous laser source, have been used in the laboratory, there is no record of them being used downhole. To date, laser light is always generated at the surface (rather than being generated downhole) and then transported downhole over a long fiber optic cable.
The surface-based approach limits the use of lasers because a long fiber optic cable is always needed. For wireline logging, conventional logging cables usually consist of seven metallic wires (six individual wires wrapped around another), housed inside of an armored cable that can support a string of logging tools weighing around 14,000 pounds, or about half of the logging cable's breaking strength. Typically, these logging cables do not have a fiber optic disposed within. Difficulties associated with disposition of a fiber optic within a logging cable include backward compatibility with existing infrastructure, the tendency of optical fiber to snap when the logging cable is stretched under load, and the difficulty of field-splicing a broken logging cable if it were to contain an optical fiber. For logging-while-drilling (LWD), such a fiber optic cable would very likely be twisted and broken by a rotating drill string. The technology does not currently exist to incorporate a fiber optic cable into the drill string, as evidenced by the fact that LWD still uses mud pulse telemetry, which is extremely slow (10 to 50 baud) instead of a fiber optic cable, which is extremely fast (125 million baud).
There is, therefore, a longstanding need for a laser system that would allow laser-based measurements to be performed downhole, in which the need for a fiber optic line to the surface is obviated, and which would admit to a greater range of practical field application and measurement techniques.